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Transistor LED Circuit

Discussion in 'LEDs and Optoelectronics' started by AcoLED, Oct 17, 2018.

  1. AcoLED

    AcoLED

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    Oct 17, 2018
    Hi,

    I am a mechanical engineer so I have a basic idea of electronics but need some expertise and clarification on an idea.

    I would like to set up LEDs in series with a transistor between each one acting as a switch. I want the transistor to have inputs of 24V and then the output voltage of the previous LED. I want the transistor to output based on the previous LED voltage until that voltage falls below the forward voltage of the next LED (typically between 2.6 and 3.0V) at which point it will output the 24V.

    I'm trying to think how this could be configured with a MOSFET or darlington pair but, again, I don't have enough electrical knowledge to determine the feasibility.

    Please let me know how this might be done or if it could be done at all.

    Thanks.
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    9,137
    1,846
    Nov 17, 2011
    Welcome to EP.

    I may not have understood your scheme completely, but it from what I understand it will not work. If you put transistors in series between a string of LEDs, the LEDs will all be off as soon as a single transistor is off. Or vice versa, for any LED to be on all transistors have to be on and consequently all LEDs will be on. There is no way to control individual LEDs by such a series connection.

    Could you please describe the effect you want to achieve in terms of LEDs and whatever control signals you have? A diagram would be best. That way we may come up with a suitable circuit.
     
  3. AcoLED

    AcoLED

    4
    0
    Oct 17, 2018
    Thank you for your reply.

    I don't want the transistor to ever switch "off." If it's possible I would like to have it switch between the two different voltages. Below is a diagram of what I am thinking. Not all of the components are or have to be exactly as they are shown but this is the idea I am hoping to achieve.

    So from the diagram, once the output voltage across the LED is less than ~3.0V, I would like to switch to the incoming 24V.

    upload_2018-10-17_11-17-16.png
     

    Attached Files:

  4. duke37

    duke37

    5,211
    718
    Jan 9, 2011
    Once the source of M1 rises above the threshold voltage of M2 then that will turn on which will turn on M3 and so on down the chain.

    The LEDs will not light as the FETs do not take gate current.
     
  5. AcoLED

    AcoLED

    4
    0
    Oct 17, 2018
    Is there any way to make a configuration (different types of transistors or components) like this feasibly work?
     
  6. BobK

    BobK

    7,599
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    Jan 5, 2010
    It is time to tell us what it is you are trying to accomplish. Your descriptions so far leave much to be desired, you are describing a proposed solution when we don't know what the problem is. Kind of hard to evaluate, though the circuit you posted, we can say with certainty, will do nothing useful.

    Bob
     
    Arouse1973 and Harald Kapp like this.
  7. AcoLED

    AcoLED

    4
    0
    Oct 17, 2018
    What I am trying to do is reduce the cuttable length on the module of an LED tape strip light. Typically the cut length is determined by the LED spacing and the cumulative forward voltage drop of the LEDs and resistors (from 24V to 0). But I am trying to see if there is a way to make each LED cuttable while still using the entirety of the 24V across the LEDs.

    Thank you.
     
  8. BobK

    BobK

    7,599
    1,641
    Jan 5, 2010
    Thanks, now it makes some sense.

    All you need to do to cut the LEDs somewhere in the middle is add a new resistor. If you can show us an extreme closeup of the strip, we might even be able to tell you how to do it.

    Bob
     
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